Mechanical principles › The complications › Striking mechanisms › Clock striking mechanisms

RACK STRIKING MECHANISM WITH DELAY

Origin and principle

The rack striking mechanism was invented in 1676 by the English Horologist (watchmaker) Edward Barlow, who sought to remedy the main flaw of the chaperon striking mechanism. His innovation consisted of replacing the independent count wheel with a stepped Snail (cam), a twelve-step Cam attached to the Hour wheel and thus making one revolution every twelve hours, exactly like the corresponding Hand. The number of strikes sounded thus depends directly on the actual position of the time, rather than on a separate Counting carried by its own dedicated striking Train (wheel train), distinct from the Movement Train (wheel train) and driven by its own Barrel.

Components and operation

A toothed rack, pivoted on its Axis and returned by a spring, carries at its end a tail that falls against the Snail (cam): the step encountered limits its travel and sets the number of teeth engaged with the striking Train (wheel train), and thus the number of blows to be Stamping. These teeth, releasing one by one, free a Lift wheel whose pins successively raise the hammer or hammers striking the bell or the Bell. As in the bonnet striking mechanism, a delay precedes the actual countdown, the Time needed for the rack to position itself against the Snail (cam) before the striking Train (wheel train) is released.

Spread and use

The decisive advantage of this principle is its direct dependence on the actual position of the hour Hand, which makes any lasting desynchronization between the displayed Time and the struck Time impossible. This mechanism gradually established itself, during the 18th century, in quality clocks, before becoming the reference for repeater watches and Grande sonnerie complications, where a Setting lever or a Pusher allows the Snail (cam) to be read on demand rather than only at the passing of the hour.